Induction melting processes and apparatus using a water cooled, segmented, copper crucible have been developed by the U.S. Bureau of Mines, for example as described in U.S. Pat. Nos. 3,775,091 and 4,058,668. These patents illustrate use of a CaF.sub.2 skull in the crucible and refractory packing material/spacers between the crucible segments to electrically isolate the crucible segments. The CaF.sub.2 type skull prevents contact between the molten metal and the crucible segments. Typically, the CaF.sub.2 is melted and solidified on the cooled inner walls of the metal crucible segments to form an insulating lining or skull between the melt and the crucible.
U.S. Pat. No. 4,738,713 illustrates an induction melting process wherein a reactive metal is melted in a water cooled, segmented copper crucible in the absence of a CaF.sub.2 lining or skull. In this patent, a refractory packing material is required between the tubular segments of the crucible to avoid molten metal penetration therebetween and subsequent skull locking.
Crucible designs of the type shown in these patents typically are based on fabrication of the segmented crucible sidewalls from a single monolithic copper forging wherein a sidewall of the forging is cut or machined to form a plurality of side-by-side segments with a relatively large gap width between the segments; e.g., a gap width of 0.010 inch or greater. As mentioned above, alumina spacers and/or refractory packing materials are provided in each gap to keep the crucible segments electrically separated and to inhibit molten metal penetration into the intersegment gaps.
Recent trends in the aerospace industry have sought to improve part service life by increasing cleanliness of the part; i.e., by reducing the quantity of harmful nonmetallic inclusions in the part microstructure. Although aforementioned U.S. Pat. Nos. 3,775,091 and 4,058,668 replace the ceramic melting pot heretofore used in conventional induction melting with the water cooled, segmented Cu crucible as a way to eliminate a known source of melt contamination (i.e., the ceramic melting pot), the use of a CaF.sub.2 type lining in the crucible unfortunately introduces another source of melt contamination as recognized in Technical Bulletin 675 "The Inductoslag Melting Process", U.S. Department Of The Interior, Bureau of Mines (1982). Moreover, the ceramic spacers and/or, refractory packing material positioned in the gaps between the crucible segments have been found to constitute still another source of contamination. For example, the ceramic spacers have been observed to break up during melting, presumably from thermal shock, with pieces of the spacers falling into the casting mold when the crucible is tilted to cast the melt into the mold.
Moreover, although U.S. Pat. No. 4,738,713 eliminates the need for the CaF.sub.2 type lining (skull) in a segmented, copper crucible, this patent still requires a high temperature refractory packing in the gap between the segments from the top to the bottom thereof to prevent molten metal penetration between the segments. Thus, a source of melt contamination is still present in the crucible in the form of the refractory material packed in the intersegment gaps.
Furthermore, water cooled, segmented, copper crucibles heretofore used are known to suffer physical damage in use. This damage is in the form of outward spreading and/or bending of the upper portions of the crucible segments resulting from loading of the metal charge into the crucible chamber and from removing of the skull from the crucible. Segment spreading and/or bending is harmful to the casting process in that the CaF.sub.2 type lining, if used, and/or refractory packing material between the segments can break loose and fall into and contaminate the melt.
There thus is a need in the art for a segmented, metal crucible which eliminates altogether the presence of a CaF.sub.2 type lining in the crucible and also refractory packing material and/or spacers in the gaps between crucible segments so as to improve melt cleanliness. There is also a need in the art to provide a segmented, metal crucible wherein outward spreading and/or bending of the crucible segments is substantially eliminated to improve crucible durability in production applications where downtime is to be minimized.